Variable resistance devices are commonly used in a variety of electrical circuits to provide an adjustable or selectable resistance. Potentiometers (pots) are common mechanically adjustable variable resistive devices. As commonly known in the electrical field, the physical position of the sliding contact or wiper along the length of the resistive coil determines the pot's selected resistance. Typically, the wiper is linked mechanically to a rotatable shaft. Turning the shaft selectively positions the wiper between the pot's clockwise rotational limit and its counterclockwise rotational limit to establish the resistance of the pot. In many applications, knowing the physical position of the wiper in relation to its mechanical limits is more important than the actual selected resistance of the pot.
Methodology and circuitry for determining the resistance for an unknown resistive device, such as the resistance setting of a potentiometer, are well known in the electrical arts. Typically the methodology consists of extrapolating a resistance for the unknown resistive device based on the known resistance of a calibrated resistive device. Fig. 1 shows a typical circuit using this conventional methodology. As shown, the circuit includes the unknown or measured resistive device Rm (such as a conventional potentiometer), a calibrated resistor Rc having a known resistance, a capacitor C, and a microprocessor MP, which provides a variety of programmed circuit functions and calculations. The known resistance of calibrated resistor Rc is stored in the memory of microprocessor MP. A reference voltage is applied across the known resistance of resistor Rc to charge capacitor C. Microprocessor MP measures the time interval to charge the capacitor from an initial voltage to a predetermine threshold voltage and stores that time interval. After capacitor C is discharged, the reference voltage is applied across unknown resistance of measured resistive device Rm to again charge capacitor C. Microprocessor MP measures the charging time interval across the unknown resistance of measured resistive device Rm and stores that time interval. Finally, microprocessor MP performs a calculation to generate a resistance for measured resistive device Rm by dividing the two charging time intervals and multiplying the ratio by the known resistance of calibrated resistor Rc.
The above circuitry and methodology can be employed with limited precision to determine the physical position of the selected setting of a mechanical variable resistive device between its mechanical limits, because the reading is subject to the inherent tolerance of the resistive device. The accuracy of the reading obtained by the conventional circuitry and methodology is directly tied to the tolerance of the resistive device itself. A more precise determination of the wiper's position can only be obtained by using a higher grade pot, which has a tighter tolerance. Consequently, a circuit and method for determining the physical position of a selected setting of a variable resistive device between its mechanical limits that is independent of the device's tolerance are desirable.